Collaborative efforts under WHO auspices to test uniformity of IPV potency determinations by monoclonal ELISA in fourteen different laboratories, demonstrated unacceptable variability between the different laboratories. Similar variability was noted in comparing potency determinations based on in-house ELISA determinations of the same coded samples. The results of this collaborative study (Wood, D.J., Heath, A.B. and Sawyer, L.A., Biologicals, 1995;23:83-94) indicated the need for further improvements in ELISA based potency determinations of IPV. Moreover, results of a subsequent WHO collaborative study comparing IPV immunogenicity in small animals (Wood, D.J. and Heath, A.B., Biologicals 1995;23:301-311) also showed an significant variablity of potency determinations from the 10 participating laboratories. In contrast to the variable results seen in the collaborative study, the monoclonal ELISA assay for poliovirus D antigen continues to be performed with precision and accuracy as an experimental test at CBER. Work is continuing in this laboratory to improve the monoclonal assay, evaluate potential polyclonal alternatives, and promote the acceptance of a suitable method by CBER, manufacturers, and other regulatory agencies.